Implantable pacemakers, cardiovertor defibrillators (ICDs), and hemodynamic monitors, are examples of implantable medical devices (IMDs) that sense cardiac electrical signals for monitoring a patient's heart rhythm. The cardiac electrical signals are sensed using electrodes positioned in or around the heart. Such electrodes may be transvenous or intracardiac electrodes for sensing EGM signals or placed subcutaneously to sense ECG signals.
Sometimes additional information relating to other physiological signals is desired for monitoring a patient. For example, information relating to the patients blood pressure, respiration, blood oxygen saturation, or patient activity may be desired. Typically, additional sensors and leads are required in order to sense additional physiological signals to derive such information. Additional sensors and other hardware can increase device cost, size and implantation procedure complexity. For example, methods have been proposed for monitoring respiration using a blood pressure signal, air flow, or thoracic impedance. A pressure sensor, a flow sensor or impedance electrodes, however, may be additional sensors that are not included in a particular medical device system. It is desirable to reduce the number of sensors and hardware needed to monitor physiological signals of interest.